1- Field of the Invention
The present invention relates to a process and a device for searching for synchronization and carrier frequency on reception of a spread spectrum digital transmission in a demodulator.
More particularly, the invention relates to a received signal which has undergone a direct modulation by pseudorandom (or PN pseudonoise) sequence in a modulator, modulation sometimes called direct sequence. The signal received in the demodulator is a signal resulting from a phase modulation of a carrier by a data signal and by a digital pseudorandom sequence signal. In this type of transmission, the power of the useful signal is considerably less than the power of the thermal noise or other interference received simultaneously. The demodulator has no prior information on either the spread spectrum sequence phase of the received signal or on the precise value of the carrier frequency of the received signal. The uncertainty concerning the value of the carrier frequency may correspond to 10.sup.-5 to 10.sup.-6 in relative value.
2- State of the Prior Art
Such a transmission process is particularly implemented for Code Division Multiple Access transmissions (CDMA): a certain frequency band centered on the carrier is shared by several communications simultaneously, and each communication occupies the entire band at all times. Such transmissions can be carried out between fixed or mobile stations by satellite, between earth radio stations and relays, for direct short-distance links between small mobile earth stations, as well as for digital links on optic fibers.
In this type of transmission, the main problem upon reception of the spread signal is the initial synchronization of the local pseudorandom sequence derived in the receiver with the pseudorandom sequence in the received signal. Traditionally, the received signal undergoes a change of frequency towards an intermediary frequency, by heterodyning with the local signal issued by an oscillator with a frequency close to the carrier frequency. The local signal is modulated beforehand by a locally generated pseudorandom sequence that is identical to that contained in the received signal but of which the phase with regard to the received sequence is to be searched for.
The synchronization between the received sequence and the local sequence is carried out by a phase drift method described by Jack K. HOLMES in the book entitled "Coherent spread spectrum systems", edited by John Wiley & Sons, New York, 1982, chapter 9, particularly paragraph 9.2.3, pages 411 to 419.
J. K. HOLMES refers to the iteration of double integration (or dwell) steps of the intermediary frequency signal resulting from the correlation between the received signal and the locally modulated signal. The locally produced sequence is applied to the received signal by successively shifting it by half-bit stepping increments (or one-half chips) with regard to the sequence in the received signal. Each shift is followed by an integration of the correlation signal over a first predetermined period after the correlation signal has been filtered and rectified. If the integrated signal exceeds a predetermined threshold, the demodulator performs a second integration during a second predetermined period, usually equal to a whole multiple of the first period, and at the end of this second integration compares the integrated signal with a second voltage threshold usually equal to the first one. If the integrated signal again exceeds the voltage threshold, the synchronization of the sequences is declared acquired.
If the threshold is not exceeded after the first or second integration, the demodulator again shifts the local sequence by a half-bit with regard to the received signal, and recommences the double integration cycle described above. Throughout the sequence synchronization search period, the control voltage of the local oscillator that supplies the clock frequency to the pseudorandom sequence generator is set at a predetermined value, such that the clock frequency is equal to an average operating value.
However, for the two decisions following the successive integrations to be properly interpreted, it goes without saying that the oscillator generating the local frequency must be set to the exact value of the received carrier. On the one hand, the signal is drowned in thermal noise, and the carrier itself is suppressed during the modulation in the modulator. On the other hand, the value of the carrier is uncertain to within a few tens of kilohertz, in practice to within more or less 100 kHz approximately e.g. when the transmission frequencies are greater than a few GHz. This uncertainty can notably result from frequency transpositions. Furthermore, to these uncertainties must be added those influenced by the Doppler effect.
It therefore appears that in the demodulator, the processing of data carried by the received signal presupposes the prior removal of two uncertainties concerning the received signal:
the phase of the spread sequence, and
the value of the carrier frequency.
Currently, in spread spectrum demodulators for autonomous receivers, i.e. that do not indirectly receive the pure carrier either by an additional transmission medium or via a frequency-shifted signal, the demodulator searches for the value of the carrier by an alternating and continuous scanning of the uncertainty range of the carrier frequency.
This scanning is obtained by applying a saw tooth or sinusoidal voltage to the local oscillator deriving the local frequency close to the carrier frequency. The scanning period is usually long, typically in the region of one second, to take response and stabilization times of the circuits involved in the synchronization search into consideration, said circuits being notably filter, detector and integrator. Repeated sequence synchronization attempts, e.g. according to the double integration method described above, are superposed on this frequency scanning. As soon as synchronization is acquired, i.e., when an integration has produced a positive result, the shifting of the local sequence is stopped. This known process has the disadvantage of being slow, and in practice requires several tens of seconds to acquire the synchronization of the sequences and the carrier frequency.